This invention relates generally to terminator devices and more particularly, it relates to an improved terminator apparatus used with a SCSI (acronym for small computer system interface) bus line for controlling the voltage level of a notch occurring in data and/or control signals transferred on the bus line so as to prevent erroneous data from being transferred.
One application where reliable data transfer becomes important is in the field of data transmission on a fully loaded SCSI bus line for communication between a plurality of data transceivers and a central processing unit (CPU) in a small computer. Typically, control signals REQ and ACK on respective request and acknowledge signal lines are used to perform a "handshake" so as to transfer the data back and forth between a target (i.e., a disk drive) and an initiator (i.e., a host computer). Each of the control signals REQ and ACK is essentially a plurality of pulses having either a high logic value or a low logic value. When the control signal REQ or ACK is asserted, it is at the low logic value. When the control signal REQ or ACK is deasserted, it is at the high logic value.
In operation, when the control signal REQ on the request line makes the transition from the asserted condition to the deasserted condition (i.e., low-to-high transition), there exists a condition on the SCSI bus line which can cause the corruption of data. At the rising edge of the control signal REQ, there will appear what is referred to as a "notch." This notch is typically accompanied by a reflection caused by stub drive cabling. The combined effect of the notch and the stub reflection will cause the rising edge of the control signal REQ to reverse its direction and "double back" before reaching the deassertion level (i.e., +2.0 volts). If this reversing control signal REQ falls below the +1.5 volt level, it may result in a "double trigger" and be interpreted as a valid request signal (i.e., another assertion), thereby causing erroneous data to be transferred.
As is generally well known in the art of computer equipment manufacturing, a termination device is typically connected to each end of an OR-WIRED SCSI bus line for supplying a fixed supply voltage with a predetermined impedance. In FIG. 1, there is shown prior art termination networks 10 and 12 connected to the respective ends of the OR-WIRED SCSI bus line 14. The termination network 10 or 12 of FIG. 1 is sometimes referred to as a "220/330 terminator." Each of the termination networks 10, 12 includes a voltage divider formed of two resistors T1 and T2 connected in series. One end of the resistor T1 is connected to an input power supply voltage TERMPWR, which is typically at +4.75 volts. One end of the resistor T2 is connected to a ground potential GND, which is typically at zero volts. The junction of the resistors T1 and T2 for the termination network 10 is connected to one end of the bus line 14, and the junction of the resistors T1 and T2 for the termination network 12 is connected to the other end of the bus line. For the SCSI bus line, the value of the resistors T1 and T2 are shown to be 220.OMEGA. and 330.OMEGA. respectively so as to provide approximately + 2.85 volts at the junction points J1 and J2 when the bus line is not active.
The OR-WIRED SCSI bus line 14 has a plurality of transceivers 16a, 16b, and 16c which are connected thereto by respective signal lines 18a, 18b, and 18c. Each of the transceivers 16a-16c includes a controller driver 20 having an open collector output (i.e., NAND logic gate type 7438) and a receiving device 22 (i.e., Schmitt trigger input type 7414). When the controller driver 20 is turned off, the signal line (i.e., line 18a) is at a high or logic "1" level which has a voltage value of approximately +2.85 volts. When the controller driver 20 is turned on, the signal line is at a low or logic "0" level since the open collector output device will pull the voltage value on the signal line down to approximately zero volts (i.e., 0.2 volts through the driver transistor Q1). For example, the signal line 18a may be used for the control signal REQ, and the signal line 18b may be used for the control signal ACK.
In FIG. 2, there is shown another prior art termination network 10a. This alternative arrangement of FIG. 2 is sometimes referred to as a "110 regulated terminator." The termination network 10a includes a voltage regulator 24 for receiving the voltage TERMPWR on line Vin and for generating a regulated voltage on line Vout of approximately +2.85 volts with 110 ohms resistors to the respective 18 signal lines DB(0)-DB(7), DB(P), ATN, BSY, ACK, RST, MSG, SEL, C/D, REQ and I/O.
The principal unsolved problem of the prior art termination networks 10 or 10a was that neither one had the capability of raising the notch above the critical "double trigger" area (between +1.5 volts and +2.0 volts). As can be seen from the curve A of FIG. 5, the signal line in the 220/330 terminator has a notch N1 occurring at approximately +1.0 volts. Similarly, there is shown in the curve B of FIG. 5 that the signal line in the 110 regulated terminator has a notch N2 occurring at approximately +1.4 volts. Thus, these prior art termination networks did not teach how the notch could be raised above the critical area.
Further, it is also generally known that the voltage level of the notch is determined by the cable impedance and the amount of current present in the bus line 14a when it is released or deasserted by the respective controller driver 20. While the SCSI specification defines cable impedance to be "no less than 90 ohms," twisted pair or "round cable" impedance is seldom found to be greater than 90 ohms and may be as low as 45 ohms.
Even though these controller drivers 20 have the capability of sinking more current, a problem occurs referred to as "metal migration" when sinking high current over prolonged periods of time. This situation arises when the request control signal (REQ) is asserted, (low logic level) and the system fails to respond with the associated acknowledge control signal (ACK). In other words, the control signal REQ is being constantly asserted and does not deassert itself. Thus, this will cause the system to "hang" until the error is cleared up by deasserting the control signal REQ. If the unattended system hangs over an extended holiday weekend, this situation could last for many days without being corrected.
There is also known in the prior art of a terminator design which utilizes a diode clamping technique for raising the level of the notch by providing current in excess of the 24 mA. However, this prior art technique suffers from the disadvantage that this extra current is uncontrolled and may vary between 24 mA to 45 mA. Further, this extra current will be provided to the controller driver indefinitely when the system "hangs" and thus may result in the destruction of the costly controller driver.
Accordingly, there has arisen a need in the computer equipment industry for an improved terminator apparatus for raising the level of the notch occurring in the data and/or control signals on the SCSI bus line so as to provide reliable and accurate data transmission. Further, it would be expedient that the terminator apparatus be capable of being incorporated internally into conventional termination networks or can be connected externally to existing systems having conventional termination networks.